This invention relates to a ultra-low refractive index and ultra-low dispersion glass having a refractive index (nd) of 1.41-1.47 and an Abbe number (νd) of 90-100 and optical elements such as lenses and prisms which are obtained by using this glass. More particularly, this invention relates to a raw material composition and a manufacturing method which can effectively prevent devitrification (precipitation of crystals) which occurs during cooling from a molten state of glass which is a serious problem in manufacture of a glass. The invention provides glass materials suitable for use as projection lenses and prisms of optical instruments represented by a camera and a projector for which a high precision chromatic aberration characteristic is required. The invention also provides optical elements and optical instruments using the optical glass of the invention.
A low refractive index and low dispersion glass material has a characteristic that change (dispersion) in the refractive index depending upon wavelength of light is small and therefore is suitable for lenses and prisms of optical instruments for which a high precision chromatic aberration characteristic is required. In the past, CaF2 crystal which is a crystalline material was used. Since it is difficult to produce a uniform and large crystal with this material and great care must be taken for processing the crystal, recent tendency is to use a glass material which is superior to a crystal material in easiness in processing and also in chemical durability. As glass materials, various glass compositions and mixed compositions are disclosed in Japanese Patent Application Laid-open Publication Nos. 60-210545, 63-144141, and 6-191876.
For realizing low refractive index and low dispersion properties in a glass material, a large amount of anion component (typically fluorine component) other than oxide ion must be introduced into glass. Since, however, glass containing a large amount of fluorine component is inferior in resistance to devitrification, devitrification occurs during cooling of a glass melt with the result that it is difficult to produce a homogeneous glass stably in a large scale production and this hinders production of the glass with high productivity.
In the above mentioned Japanese Patent Application Laid-open Publication Nos. 60-210545 and 6-191876, improvement in resistance to devitrification is discussed but, in these literature, consideration is made only for improvement in resistance to devitrification by holding the glass during a short period of time of one to two hours in the vicinity of the glass forming temperature. No sufficient consideration has been made in these literature for improving resistance to devitrification during a long period of time which can be applied to a continuous melting and glass-forming production system which has high productivity.
Japanese Patent Application Laid-open Publication No. 9-142875 discloses a method of manufacturing a fluoride complex phosphate optical glass for the purpose of preventing forming of devitrification and striae. In this method, however, if it is desired to obtain a large-size glass material while maintaining a high glass casting temperature, glass forming has to be made in a low viscosity region and this will cause difficulty in controlling the flow amount of glass precisely and, as a result, a non-uniform striped portion (striae) tends to be produced by the flow of glass. Besides, since glass has small heat conductivity, the inside portion of the glass which is difficult to cool compared with the surface portion of the glass has longer time during which the glass is subject to devitrification. Thus, there is likelihood that devitrification occurs in the inside portion of the glass and, therefore, improvement in this literature against devitrification cannot be said sufficient.
It is, therefore, an object of the invention to provide a ultra-low refractive index and ultra-low dispersion optical glass having a refractive index (nd) of 1.41-1.47 and an Abbe number (νd) of 90-100 which can be obtained stably by using a commonly used melting method without using a special material or a special melting crucible or a special melting method.